Quantitative detection of fluorescent microplastic particles with a full-format camera for atmospheric transport wind tunnel studies.

, Esders Eike Maximilian, Babel Wolfgang, Thomas Christoph.

Atmospheric transport of microplastic particles (MPP) is an emerging topic and we need new methods to improve our understanding of suspension, transport and redeposition of MPP. The atmosphere is not a major source or sink for MPP, but recent findings demonstrate the atmosphere as an important and fast local and global transport pathway. Our objective was to develop a quantitative visual detection method for MPP which can be applied in wind tunnel experiments. Our goals were to A) detect the total number of MPP suspended on a glass carrier, B) detect and exclude clusters of particles and their formation or disbanding, and C) filter noise originating from dust or fragmented particles. A full-frame visual camera was employed in combination with fluorescent MPP excited by ultraviolet light. Images were analysed using a newly developed software: the image gets reduced to its green channel, followed by an intensity filter which sets green pixels below a carefully selected intensity threshold to zero, and to one above it. This 1-bit image was then processed for the total count of all MPP using their size, aspect ratio, and circularity. Given the optical configuration MPP with a diameter of 53 µm are represented by about 200 pixels. We present a robust counting method that can quantitatively detect MPP with high precision (SD of 0.31%) at a small processing time of approximately 4s for an image containing several hundred particles. Size, aspect ratio and circularity can be used effectively to filter noise. The method enabled a robust quantitative detection of fluorescent particles of different diameters down to 53 µm in recent wind tunnel suspension experiments for which we will show the results. Future work will include adjusting the detection method for examining the atmospheric transport of much smaller MPP, different shapes, and a variety of flow conditions.

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